Planetary screw drives (PSDs) (also referred to as planetary roller screw drives) have been the state of the art for many years, and are described, for example, in DD 0277308 A5. A planetary screw drive is known, for example, from published patent DE 10 2010 047 800 A1, which is contained in a hydrostatic actuator in the form of a hydrostatic clutch actuator in order to convert a rotary motion produced by means of an electric motor to an axial motion. A planetary screw drive having a screw spindle and having a nut situated on the screw spindle and having a plurality of planet gears distributed around the circumference, positioned between the screw spindle and the nut, which are arranged so that they can roll on the inside circumference of the nut as well as on the outside circumference of the screw spindle, is known from printed publication DE 10 2010 011 820 A1. In this solution, a pre-stressing device is provided for the planet gears, wherein the nut has two parts that are movable axially relative to each other, and wherein the pre-stressing device has a spring element that is spring loaded against the one nut part. The nut assumes two functions: on the one hand it is part of the transmission, and on the other hand it is part of the pre-stressing device.
It is also known (see DE 10 2011 088 995 A1) to perform an absolute measurement to register the slip of a PSD having a screw spindle and having a spindle nut situated on the screw spindle and having a plurality of planet gears distributed around the circumference, where a sensor element that is positioned axially immovably relative to the spindle nut registers an axial displacement of the screw spindle and the spindle nut relative to each other; the spindle nut being supported on a housing that has the sensor element, so that it can rotate around the spindle axis.
A yet unpublished application describes a disengaging system for a clutch of a motor vehicle, wherein while using a drive a piston that is actuated is supported so that it is movable axially in a housing by means of a PSD, while the position of the piston is ascertainable using sensor technology.
The known systems are of relatively complex design, since a pre-tensioning must be realized and a change in the pitch of the PSD system due to slip makes a distance sensor necessary for exact positioning.
For the boundary condition that the pitch in the ring gear nuts and on the planetary rollers 3 is zero, the following boundary cases result:
Boundary case 1: when there is sticking between spindle and planetary rollers, no advance/axial movement occurs relative to the spindle when the latter rotates, so that 100% slip must be registered between planetary rollers and ring gear nut and the efficiency is zero.
Boundary case 2: when there is sticking between planetary rollers and ring gear nut, a spindle pitch of 1:1 with 100% slip between planetary rollers and spindle is realized, so that the efficiency is very low.